Type guard functions seem to only set the type when used directly

[jvanoostveen]

TypeScript Version: typescript@beta (2.0)

Code

// Setup
interface A {
  value: string;
}
interface B {
  property: string;
}
type AorB = A | B;

let isB = (obj: any): obj is B => {
  return obj.hasOwnProperty('property');
};
// Setup end


let output = (obj: AorB) => {
  if (isB(obj)) {
    console.log(obj.property); // <-- works

    let values = (new Array(100)).map(a => obj.property); // <-- Property 'property' does not exist on type 'A | B'
    console.log(values);
  }
};


let someObj: B = { property: 'value' };
output(someObj);

Expected behavior:
obj.property within the map should still know its type. This works fine in pre-2.0.

Actual behavior:
The compiler does not recognise property as part of obj within the map.

[jvanoostveen]

In some of our existing code I get the following error on similar structures:
Error TS2339: Property 'value' does not exist on type 'never'.

[kitsonk]

This is a dupe of #7719 and so many others. Specifically the comment is here as to why it is the way it is. The team is considering allowing parameters to be declared as const to indicate they will not be changed within a function.

[jvanoostveen]

Looks like I searched for the wrong terms :). Continue on the referenced issue then.